libgd, the graphics backend of php. It's an ancient pile of shit yet the "maintainers" refuse An help, contributions and improvement as good as they can. The most popular example is emoji support on their text rendering. Questions on SO, bug reports to libgd and php go back at least 10 years, yet no interest by the devs to either fix it or accept any help in doing so.

A couple years ago I started investigating and it starts with the fact that their utf8 decoder accepts no more than 3 bytes per character, thanks to an ifdef. There's code to handle 4, so you change the ifdef and then it doesn't compile anymore because code rot. Fixing that you see the correct code points going into freetype but then something else I forgot about breaks. Then I saw there were already a couple patches on the mailing list and checking their github just now I see at least one PR with a bunch of back and forth and another couple issues with mostly no reaction or at best a few lame excuses.

If you could donate negative money to a project this would be a prime candidate. Sure, move fast and break things is bad, but this is the opposite of that, and not good either.